1. Field of the Invention
The present invention is related to the field of communications through networks, and more specifically to devices, softwares and methods for prioritizing between voice data packets for discard decision purposes.
2. Description of the Related Art
Networks, such as the internet, are increasingly used for communications. The Internet Protocol (IP) has been developed for communications through the internet.
Like other packet network technologies, the Internet Protocol (IP) was initially designed to transport data in a reliable and robust way. IP ensures that all data is delivered, even though in an asynchronous mode.
It was only later that networks started being used for transporting video data and voice data. The latter takes place using a Voice over Internet. Protocol (VoIP). VoIP ensures that voice data are retransmitted in real time. If any are lost, they are not retransmitted. If any arrive too late, they are not incorporated in the playout.
Since the initial design of IP focused on transferring data asynchronously, it largely ignored considerations such as Quality of Service (QoS) for VoIP. Accordingly, as VoIP evolves, more and more efforts are being made to ensure an acceptable QoS over networks, such as IP networks. Schemes are developed, such as Resource Reservation Protocol (RSVP) and Class Based Queuing (CBQ). These schemes also address how to handle the difficult situations, such as congestion in a network, etc.
CBQ techniques define several logical queues in a memory buffer. Each queue is assigned a priority. As each incoming packet is received, it is sorted into one of these queues, where it awaits retransmission. Sorting is based on a number of criteria, such as the Type of Service (ToS) field of the packet, or a combination of source address, destination address and port. Then a scheduler selects one of the queues based on its relative priority, and forwards packets from it to the interface physical queue, from where they are routed.
Since voice and video data packets have to be retransmitted at real time, they tend to be given a higher priority than data. And that is in networks that were initially designed for data, instead of voice or video.
When there is a congestion in a network due to voice traffic, some of the packets carrying voice data are dropped. This means they are discarded, without being retransmitted, which guarantees they will not arrive for playout.
Discarding voice packets takes place randomly, depending only on the condition of network congestion. The CBQ techniques do not differentiate between packets within a queue.
The speech is reconstructed for playout. Packets that got dropped along the way are typically reconstructed by interpolation from their neighboring packets that did arrive.
The problem is that, for reconstructing speech, some packets are more perceptually important than others. But their relative importance is not accounted for in the discard decisions of a congested router. Accordingly, an important packet has an equal chance of being discarded as a less important packet. When important packets are discarded, interpolation aggravates the fact that they are missing. Thus the reconstructed voice can have a poor quality, even if the network is only mildly congested.